Materials Performance

JUN 2016

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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15 NACE INTERNATIONAL: VOL. 55, NO. 6 MATERIALS PERFORMANCE JUNE 2016 Biof ilms with a mixture of microbe spe- cies form naturally on many metal sur- faces, and it is these biof ilms that are most likely to be linked to increased localized corrosion. The new methodolog y being devel- oped involves an electrochemical test conducted on CS coupons (either with or without the presence of bacteria) exposed to a semi-solid agar electroly te mixture comprised of a low concentration of sodium chloride (NaCl) and a peptide- based nutrient broth. Pseudomonas f luo- rescens, common rod-shaped bacteria that colonize soil, water, and plant sur- face environments and are highly likely to be present in the conditions around a pipe, are used in the test. These bacteria have been previously implicated as con- tributing to localized corrosion of CS. Microbiological-grade agar is used to simulate both the physical structure and chemical components of soil more closely than can be done with aqueous solutions, which can be made to mimic the chemical composition of soil, the authors say, but do not take into account the physical structure of soil, which plays a major role in electrochemical interactions between soil, microbes, and the steel. For the tests conducted by the authors, sterilized CS coupons were swabbed with a P. f luorescens culture. A standard three-electrode polarization cell was constructed with a 500-mm 2 platinized titanium mesh counter elec- trode, a standard calomel reference elec- trode (SCE), and a polished CS sample (either with or without bacteria swabbed onto the surface) as the working elec- trode. An agar electroly te solution (4 g/L of agar, 12.5 g/L of nutrient broth, and 2.5 g/L of NaCl in distilled water) was poured into the completed cell and then allowed to set for ~2.5 h. The open circuit potential (OCP) was then measured for durations of 3 and 24 h. "Because we were working with bacte- ria, we had to maintain a sterile environ- ment until we added our bacteria to the system," says Spark, noting that the envi- ronment needed to remain aseptic so the authors knew the effects they were seeing were due to the bacteria they had intro- duced and not something from the air in the lab. "We also had to be much more careful with cooling the agar before we added it to the system so that the bacteria didn't go through thermal shock, which Time vs. OCP data for 3- and 24-h relaxed potential holds for samples with and without the addition of bacteria. Continued on page 16 Information on corrosion control and prevention

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